1. Field of the Invention
The present invention relates to an abnormality detection apparatus for a rotary type absolute encoder which includes a rotor portion and a stator portion disposed independently of each other, and in which information indicating an absolute position recorded in the rotor portion is read by an absolute sensor disposed in the stator portion.
2. Description of the Related Art
Heretofore, drive systems in which reduction gears including a worm gear or the like are combined with a servomotor have been used in the rotary shaft of circular tables or the like of machine tools. A drive system employing reduction gears, however, suffers from a problem of decreasing precision attributed to backlash and the limitation of the enhancement of a rotational speed. Recently, therefore, direct motor drive systems in which a built-in motor is incorporated and which more easily provides a higher precision and higher speed have come into use for the rotary shaft.
In general, a rotary type absolute encoder is used in the rotary shaft of the direct motor drive system for the purposes of performing magnetic pole position detection, speed detection and position detection which are necessary for the control of the built-in motor.
One type of rotary type absolute encoder is that disclosed in, for example, Japanese Patent Publication JP-A-2005-61907 (Patent Document 1). In this rotary type absolute encoder, a rotor portion bears an absolute code in which any continuous N bits at any desired rotational position are all different, and a stator portion reads the code of the N bits in the rotor portion, so as to detect an absolute position.
With the abnormality detection apparatus of this rotary type absolute encoder, if even one bit of the code of the N bits should be misread, it is possible that an erroneous absolute position might be determined without successfully detecting any error (abnormality). Even in such a case, however, because the rotor portion is rotated, whereby the error of the absolute position can be immediately detected. Meanwhile, when the abnormality of the absolute position is made detectable in a static state in order to heighten the reliability of the absolute position, it has been necessary that the stator portion be capable of reading an absolute code of about 2N bits as redundant bits for error detection. When an absolute code as large as 2N bits must be read, a sensor for reading the absolute code must be arranged on the surface of the stator portion in conformity with the curvature of the rotor portion. Further, stator portions must be individually prepared for the respective sizes of rotor portions. Because of these requirements, a problem of difficulty of handling a wide range of rotor diameters with one type of stator portion has remained, as stated in Patent Document 1.
Additionally, a method wherein two absolute encoders are mechanically disposed has been employed as a method for enhancing the reliability of the absolute encoder. This method, however, is problematic in that the required encoder installation space is basically doubled, while associated costs are double, or even more.